1. Field of the Invention
The present invention relates to a base film of polyvinyl alcohol and more particularly to a base film of modified polyvinyl alcohol as well as its preparation method and a polarizer prepared from the base film of modified polyvinyl alcohol.
2. Description of the Prior Art
Polarizer is one of the essential important components in a liquid crystal display (LCD), and its function is to convert natural light into polarized light, in order to cooperate with liquid crystal molecules twisted to control light passing and presentation of color signals. An existing polarizer used in a LCD is mostly made of a highly oriented high polymer such as a base film of polyvinyl alcohol (PVA) as its substrate, dyed with various types of dichroic dyes, extended under a certain humidity and temperature, and then a layer of triacetate cellulose (TAC) film with high light transmission rate, excellent waterproof characteristic and a certain degree of mechanical strength is composited on each of two sides of the PVA base film, so as to prepare the polarizer. In comparing to polarizers dyed with organic dichroic dyes, iodine polarizers have become the mainstream polarizers for LCDs because of their polarization characteristic of wider range of wavelengths, higher light transmission rates and better price-performance ratios. Nevertheless, iodine polarizers are comparatively less resistant to heat and water. As liquid crystal displays are commonly applied to many products, higher durability for polarizers are also demanded. Therefore, modification of PVA base films for development of polarizers with excellent optical and durability performances for LCDs is a technical difficulty encountered currently.
Firstly, a PVA base film is swelled, soaked and dyed in iodine, and extended uniaxially; then a layer of triacetate cellulose (TAC) film is composited on each of two sides of the PVA base film, so as to prepare an iodine polarizer. PVA is a linear high molecular polymer with a plurality of —OH groups of strong polarity evenly distributed in a long molecular chain as shown in FIG. 1, therefore it has relatively strong hydrophilic property, and is less water resistant and less stable. Furthermore, iodine molecular structure is easily damaged under high temperatures and humidity, so that iodine polarizers are less humidity and heat resistant with poor mechanical performance. Generally, they may only meet testing conditions of 80° C.×500 hrs or 60° C.×90% RH×500 hrs, and may easily become warped and peeled off, resulting in limiting their application range.
Referring to FIG. 1 which shows a diagram of a molecular structure of a PVA base film of an existing iodine polarizer, wherein a TAC protection film of the iodine polarizer is not shown. A plurality of OH groups with strong polarity and hydrophilic property is evenly distributed on a surface of a polyvinyl alcohol polymer 3 of the PVA base film. And a structure of iodine molecules 2 may be easily damaged under high temperatures and humidity. Therefore, the PVA base film of the iodine polarizer is relatively less stable under high temperatures and humidity.
In order to improve the heat and humidity resistances of the PVA base film of the iodine polarizer, the PVA base film needed to be modified or cross-linked. Currently, a method for improving the heat and humidity resistances of the PVA base film is to add a second constituent which is a material to be cross-linked with the hydrophilic —OH groups in PVA. For example, both China patents published No. 1979231 A and 101281267A employ dicarboxylic acids and boric acids to have the PVA base film cross-linked. Nevertheless, boric acids increase the toughness of the PVA base film which will limit its extension percentage during a stretching process. Others use vacuum coating or ion sputtering method to have silicon dioxides (SiO2) plated on the PVA base film in order to improve its heat and humidity resistances, but costs for both coating methods of silicon dioxide film are relatively high, and a problem with compatibility between silicon dioxides and the PVA base film also arises.
Therefore, a new PVA base film is required in the hope that it features relatively higher heat and humidity resistances, stability and mechanical performance, and at the same time with lower defective percentage of polarizers and have anti-glare function.